The UN Security Council delayed until Thursday a vote on a Western-backed resolution that threatens Syrian authorities with sanctions and is aimed at ending the 16-month conflict, diplomats said on Wednesday.

International envoy Kofi Annan earlier had requested that the vote, which was planned for later on Wednesday, be postponed because he wanted to "ensure all efforts were made for UN Security Council to speak with one voice and agree on concerted pressure," Britain's UN mission said on Twitter.

UN chief Ban Ki-moon urged the council to "shoulder its responsibility and take collective and effective action."

"Time is of the essence. The Syrian people have suffered for too long. The bloodshed must end now," Ban said in a statement.

The 15-member Security Council needs to decide the future of a UN observer mission in Syria before its mandate expires on Friday. The council approved the mission's deployment to monitor a failed April 12 ceasefire under Annan's six-point peace plan.

But the permanent five veto-wielding members of the Security Council -- the United States, Russia, Britain, China and France -- are divided over whether stronger action should be taken.

Britain, the United States, and France, along with Germany, want the council to threaten Syria's President Bashar al-Assad and his government with sanctions in a bid to halt the violence, but Russia and China are against such a move.

Russia, a key ally of Syria, has refused to engage in negotiations on the Western-backed resolution that would extend the UN observer mission in Syria for 45 days and place Annan's peace plan under Chapter 7 of the UN Charter, diplomats say.

Chapter 7 allows the council to authorize actions ranging from diplomatic and economic sanctions to military intervention. U.S. officials, however, have said they are talking about sanctions on Syria, not military intervention.

"We had a useful discussion but I wouldn't call it a negotiation," U.S. Ambassador Susan Rice told reporters after further talks between the permanent five members. When asked if she expected any new proposals from Russia, she said: "No."

Sliding into chaos

Russia has said it would block the Western-backed resolution because it does not believe it should be placed under Chapter 7. Russia and China have already twice vetoed UN Security Council resolutions designed to pressure Assad to halt the violence.

Russia has also put forward a resolution to extend the UN mission for 90 days, but it does not contain a threat of sanctions. It was not clear when or if Russia planned to put its draft resolution to a vote.

"We have very clearly told our Russian and Chinese friends 'we are ready to negotiate,' French UN Ambassador Gerard Araud told reporters. "The framework is well defined, it's a Chapter 7 resolution with a threat of sanctions."

The Western-backed draft resolution specifically threatens Syrian authorities with sanctions if they do not stop using heavy weapons and withdraw troops from towns and cities within 10 days of adoption by the Security Council.

"We want to give diplomacy a chance, but of course there has to be meaningful engagement on the side of Russia and China with our resolution," Germany's UN Ambassador Peter Wittig said.

"Syria is sliding into chaos. We have been warning about such a development all along. The responsibility for that development falls squarely with the Assad regime," he said.

A suicide bomber killed Syria's defence minister and two other top military officials in Damascus on Wednesday, security sources said, the worst blow to Assad's high command during the rebellion.

"We condemn all kinds of terrorist actions," China's UN Ambassador Li Baodong told reporters, adding that he hoped the council could reach consensus on what action to take on Syria.

UN Secretary-General Ban also condemned the attack and said he was alarmed at the intensifying violence in Syria.

Ban was also "concerned about reports of the continued use of heavy weapons by the Syrian security forces, including in the Damascus area, against civilians, despite repeated government assurances that such weapons would be withdrawn."

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